In conventional motor control units, for instance, as disclosed in Japanese Laid-Open Patent Application No. 280751/'86, a unit which generates a rotating magnetic field through repetitive energizations of the exciting coil and interruptions thereof by timing a transistor ON/OFF is known for starting or otherwise controlling a motor.
When a motor is overloaded, its speed will be lowered until it eventually locks. Then, the exciting coil of the motor is subjected to a current approximately three times greater than the current supplied when the motor is running under a normal load. This results in undue heating of the motor which causes the motor to burn, deteriorate, shorten in life and other similar malfunctions.
Therefore, a motor control unit provided with an anti-burning device has been used which, for instance, detects the temperatures of semiconductors constituting a control circuit. When the temperature of such semiconductors have risen above a preset temperature, the supply of current to the motor is stopped.
Furthermore, various motors used as power source have different resistance values of the exciting coil according to the rated voltage and the torque generated. Hence, the amount of current flowing through the exciting coil is different. Even with motors having the same amount of current flowing under an overloaded condition varies according to differences in the resistances of the exciting coils, environmental conditions, etc.
However, a motor control unit provided with a conventional anti-burning device, as mentioned above, is arranged so that the current supply to the motor is stopped upon detecting a rise in temperature of the semiconductor elements constituting the control circuit. Hence, with a motor having a relatively high amount of working current, it is possible for the anti-burning device to stop the supply of current to the motor even if the load on the motor is not extremely large.
Meanwhile, in a motor having a relatively low amount of current flowing even when it is overloaded, it is possible that the temperature of the semiconductor elements constituting the control circuit may not reach the preset temperature even if the motor is overloaded. In such a case, the anti-burning device may fail to act or act too late which may result in a failure to ensure the motor against burning, deteriorating, shortening in working life, etc.
To cope with this problem it is possible to modify the detection sensitivity and detect the rise in temperature of the control circuit according to the type of the motor to be controlled, but this is bound to result in the loss of the general purpose features of such a motor control unit.
In another function required for this kind of motor control unit, after the current supply to the motor is stopped by actuation of the anti-burning device, it is necessary to resume the current supply when the load on the motor has been relieved. In such a case, as mentioned above, it is possible that an anti-burning device for stopping the current supply to the motor through the detection of a rise in temperature of the semiconductors constituting the control circuit. It is conceivable to resume the current supply to the motor after stopping the current supply to the motor, the temperatures of the semiconductor drop below the predetermined temperature.
However, even if the load is relieved immediately after stopping the power supply to the motor, a relatively long amount of time is required before the temperature of the control circuit is lowered and the current supply is resumed. Also, the amount of time until the power supply resumes will vary depending on the environmental conditions such as the atmospheric temperature. Even when power supply is resumed, if the overload condition remains, it means that the overcurrent will continue to blow until the temperature of the control circuit increases and actuates the anti-burning device, which may cause a deterioration in performance and a shortening in life of the motor.